Deformation textures and mechanical behavior of a hydrated amorphous silica formed along an experimentally produced fault in chert

We conducted a series of friction experiments on chert at intermediate to high slip velocities ( V = 0.87 − 104 mm/s) and at low normal stress ( σ n = 1.5 MPa) to better understand the process of fault weakening by silica‐gel formation, as identified in previous friction experiments on quartz‐rocks. Fault weakening in chert samples occurred in association with the formation of a 0.1‐mm‐thick fault gouge layer that contains a thin, foliated layer of fine‐grained material and a cataclastically fragmented zone characterized by clast‐matrix texture. MFT‐IR and XRD analyses reveal that the fault gouge consists of a mixture of hydrated amorphous silica and quartz grains. The mechanical behavior of the fault gouge at a small magnitude of strain ( γ = 0.008), as examined independently of the friction experiments using a rheometer, is characterized by a negative dependence of shear stress on strain rate with a notable hysteresis behavior.